This invention relates generally to an apparatus and process for transfer-coating a low-viscosity polymerizable liquid from a curtain-coated applicator means to either a thin, delicate open-mesh or batte web which is embedded in a uniform liquid layer of substantially the same thickness as the web, and the liquid is then polymerized; or, to a continuous web forming a backing for the liquid.
This invention particularly relates to the production of a thin, web-reinforced sheet or film useful in applications which capitalize on the unique properties of a hydrophilic interpolymer. Monomers which are polymerized to form the interpolymer may be coated on a thin film of corona-treated polyolefin, specifically polyethylene, which is wetted by the monomers. Alternatively an open-mesh web or net of a polymer which is not wetted by the monomers confines them so as to envelop the net which provides reinforcing for the interpolymer.
Such an interpolymer may be prepared by the copolymerization of a low-viscosity liquid mixture of monomers comprising (a) an unsaturated carboxylic acid and (b) a salt of the unsaturated carboxylic acid in which the majority of the carboxylic groups has been neutralized with an alkali metal hydroxide or ammonium hydroxide, in the presence of one or more of the following monomers selected from the group consisting of (i) a higher alkyl (meth)acrylate (that is, acrylate or methacrylate), (ii) a lower alkyl (meth)acrylate, (iii) an alpha-olefin having from 6 to about 18 carbon atoms, styrene or a substituted styrene, (iv) a cross-linking agent which contains two or more ethylenic unsaturations, and (v) 2-hydroxyethyl methacrylate or dialkylaminodialkyl (meth)acrylate. These interpolymers are known to absorb water and body fluids such as urine and blood, rapidly, as disclosed in U.S. Pat. No. 4,167,464, and copending patent applications Ser. Nos. 183,616; 389,822; and 427,325, the disclosures of which are incorporated by reference thereto as if fully set forth herein.
The problem is that utilization of these desirable properties of the interpolymer is thwarted by the difficulty of incorporating it in a product with a suitable woven fabric or non-woven batte made from textile fibers because of the peculiar, particular properties of a mixture of monomers from which the interpolymer is formed. The term "mixture of monomers" is used herein even when the polymer is formed from (a) acrylic acid and (b) an alkali metal or ammonium acrylate, and such polymer is referred to herein as an interpolymer as these monomers are distinct from each other.
By "fibers" we refer to filamentous elements which are organic natural fibers or synthetic fibers such as those produced from spinnerets which fibers may be used to form a batte of random or oriented fibers, or to form yarn which in turn may be woven into a fabric or netting. In addition to the peculiar property of low-viscosity, the liquid has a high proclivity to polymerize in the nozzle of a spray gun from which it is sprayed, making spray-coating a web of fabric impractical.
In practice, the interpolymer is prepared by photopolymerizing, preferably in the presence of a photoinitiator and a dispersant, (a) from about 10 to about 60 percent by weight (wt. %) of acrylic acid, (b) a salt formed by neutralization of 40 to 90% of the acrylic acid with an alkali metal hydroxide or NH.sub.4 OH, and optionally, (i) less than 25 wt % of a higher alkyl (meth)acrylate wherein the alkyl group has from 10 to about 30 carbon atoms; (ii) less than 30 wt % of a lower alkyl (meth)acrylate wherein the alkyl group has from 1 to 8 carbon atoms, a minor amount, or none, of the lower alkyl (meth)acrylate being replaced by (meth)acrylic nitrile or amide; (iii) hexene or styrene; (iv) less than 10 wt % of a cross-linking agent having two to six ethylenically unsaturated groups which can be copolymerized with acrylic acid by UV radiation; and (v) from 5 to 35 weight percent of 2-hydroxyethyl methacrylate or dialkylaminoalkyl (meth)acrylate in which each alkyl of the dialkyl groups has 1 to 8 carbon atoms, and the other alkyl group has 2 to 6 carbon atoms; so that the polymerized film formed contains in excess of 25% water.
By a low-viscosity liquid mixture we refer to a liquid which has a viscosity in the range from about 2 to about 100 centipoises (cp), more specifically from about 8 to about 30 cp, which is referred to herein as being `non-viscous` because the mixture defies being coated, so as to form a continuous liquid layer, on many a substrate surface if it is not either `etched` or especially formed so as to restrict the run-off of the liquid.
For example, even with the use of a surface active agent or dispersant, the mixture of monomers does not wet, but runs off, a laminar substrate of nylon, untreated polyester or polyolefin. With a dispersant included in the mixture, it wets paper, cotton, wool or polyester surfaces, but only with difficulty. A woven fabric of nylon or polyester fibers, such as a net, when dipped in the mixture and held so as to hang in a vertical position, retains almost none of the mixture even when the interstices of the net are only about 1 mm in diameter. Even a non-woven batte of nylon or polyester fibers is wetted with difficulty. The problem can best be visualized by comparing it to the problem of coating a net with concentrated sulfuric acid at room temperature, so that the acid uniformly fills the interstices of the net.
To combat this problem, the '464 patent teaches the use of a wicking aid such as a glycol, to improve the rate at which the liquid moves along the fibers or film being coated, if the liquid wets them at all. However, even with a wicking aid, it is difficult to wet a horizontally supported reinforcing nylon net or non-woven nylon batte with the mixture of monomers, and we know of no prior art teaching as to how such reinforcing might be wetted sufficiently to embed it in a thin, substantially uniform liquid layer of the mixture. By "thin" we refer to a thickness from about 2 mils to about 25 mils thick, and by "substantially uniform" we refer to such thickness .+-.20%.
The addition of cross-linking agents may improve the strength of the polymer formed by the action of actinic radiation, but the use of a cross-linking agent in an amount sufficient to improve the wettability of the reinforcing material adversely affects the desirable properties of the polymer formed. Thus, to the extent such wettability may be improved, it is accomplished with a choice of dispersants.
The '464 patent teaches that the interpolymer has particular utility in the disposable non-woven industry where there is a need for polymers which will absorb and retain water and physiological fluids. In a specific example, it teaches a disposable diaper in which fibers of the interpolymer, or non-woven agglomerates of its fibers may be included; or, in which diaper a film of the interpolymer may be used between a fluid-impermeable outer plastic layer and inner fluffy absorbent layer of the diaper. There is no teaching of the use of a reinforced film of the interpolymer, with the reinforcing web embedded intermediate the upper and lower planar surfaces of the film, because there did not exist a practical method of making such reinforced embedded film for use in a marketable article of commerce. The problem of coping with the peculiar physical properties of the mixture of monomers in which the reinforcing material is embedded by a method which forms a thin continuous film of the interpolymer, had yet to be solved.
Numerous prior art methods have been devised for coating low viscosity liquids on substrates, each of which methods is directed to the solution of particular problems presented by the properties of the fluid to be coated and the substrate upon which it is to be coated. Brush-coating of a web is generally restricted to the application of relatively thick coatings of relatively viscous fluids having a viscosity greater than 100 cp, is known to leave undesirable streaks and other non-uniformities with coatings less than 25 mils thick, and is therefore avoided in the coating of low-viscosity liquids. Further, when the web is an easily unraveled batte, or an easily distorted net of a filament from about 1 mil to about 10 mils thick, maintaining the integrity of the web becomes a critical consideration.
Most methods for coating low-viscosity liquids in which a web is embedded are directed to the production of textiles or textile-like fabrics which are unrelated to the web-reinforced articles of this invention both in appearance and in function. Moreover, these methods are directed to the curtain-coating of a controlled thickness of a polymerizable non-viscous fluid, in the range from about 2 mils to about 25 mils, on to a travelling web; very few of these are directed to the further problem of stripping or delaminating the reinforced polymer film from a substrate not wetted by the fluid and upon which substrate the polymer is formed. To our knowledge, no prior art method useful for coating a controlled thickness of a low-viscosity liquid teaches the use of a stationary curtain-coated (with the liquid) applicator curtain with the specific purpose of transferring a thin coating to a moving web.
Stated differently, knowing that a thin net or batte (web) cannot be coated uniformly with a thin layer of the non-viscous mixture of monomers by being dip-coated, or spray coated, or coated from a transfer roll, knife over-roll, squeeze roll, or reverse roll such as are conventionally used for relatively low-viscosity liquids, and knowing the mixture cannot be coated on the thin web by rotagravure coating as is routinely done with low-viscosity inks and the like, the particular problem is to embed the web, which is wetted by the mixture, in a substantially uniform liquid layer from 2 to about 25 mils thick while the web is supported on a substrate. In an additional step, after the woven or non-woven web of reinforcing material is embedded in the non-viscous mixture, then polymerized, the reinforced interpolymer film must be parted from the substrate upon which it is polymerized.
The uniform flow of liquid from weir-like structures including a reservoir from which a moving web is coated, the direction of movement of the web being transverse to the longitudinal axis of the reservoir, has been considered generally difficult to control. Conventional reservoirs with a weir, and a curtain supplying liquid across the weir, are known to produce non-uniform flow across the length of the weir, which proscribes their use where uniformity is essential. Such reservoirs are also known to be sensitive to pulsing of the liquid flowed into the reservoir, which proscribes their use where a thin film is to be curtain-coated on a web. Accordingly, the conventional weir-like structure has been replaced with structures such as those disclosed in U.S. Pat. Nos. 3,365,325 and 3,369,522.
More recently, U.S. Pat. Nos. 3,587,527; 3,911,174; 4,019,906; 4,075,976; 4,178,221 and 4,197,812 address particular curtain-coating problems, and how they may be solved by obviating the conventional weir-type structure, the tendency of droplet formation with a curtain which does not terminate in a knife-edge, and other problems. Though none of the disclosures is particularly directed to curtain-coating with a low-viscosity liquid, and, examples provided are of liquids having substantially higher viscosities than 100 cp, it is clear that one skilled in the art when faced with the particular aforementioned problems, would not be led to use a curtain coated web as an applicator from which the liquid is to be transferred to another web.
Reverting to the '464 patent, it is stated that a film of the monomer mixture can be spread on the surface of a suitable substrate to the desired thickness, e.g. 1 mil to 25 mil, and then subjected to UV radiation for a short time, e.g. 1 second to several minutes. Substrates mentioned are Mylar, polyethylene, and paper, inter alia, but it was not recognized that the monomer mixture wetted only etched polyethylene and that non-corona treated Mylar.RTM. was not wetted at all. Therefore, on such non-wettable surfaces, the mixture of monomers cannot be spread by conventional methods such as with a Boston-Bradley adjustable blade or by spraying.
Further, since from a practical standpoint, the polymer is formed by exposure to UV radiation at a sufficient intensity to effect a cure in a short time in the range of from about 20 sec to about 40 secs, it is essential that the reinforcing material be substantially permeable to the UV radiation. By "substantially permeable" we mean that not enough UV radiation is absorbed by the mass of fibers to deleteriously affect their properties or those of the interpolymer formed. For example, paper and natural organic fibers get too hot to be useful as reinforcing material.
Thus, to our knowledge, the problem of polymerizing a uniform, thin, continuous liquid layer of a non-viscous mixture of plural monomers by exposure to ultraviolet radiation, or electron beam radiation where the polymer formed is to be reinforced with a substantially uv-permeable reinforcing material, is a novel problem which has not been successfully solved.